1. Field of the Invention
The present invention relates to a contactless (wireless) method of supplying power that disposes a transmitting coil and a receiving coil in close proximity to magnetically couple the coils to supply power from the transmitting coil to the receiving coil by magnetic induction, and in particular to a contactless method of supplying power that detects foreign objects on the power supply stand (pad, platform, stage, cradle).
2. Description of the Related Art
A contactless method of supplying power that places a portable device housing a receiving coil on a power supply stand housing a transmitting coil and transmits power from the transmitting coil to the receiving coil has been developed. (Refer to Japanese Laid-Open Patent Publication 2008-17562.)
In the contactless method of supplying power of the related disclosure, a charging pad serves as the power supply stand, a battery powered device serves as the portable device, and power is transmitted from the charging pad to the battery powered device to charge a battery internal to the battery powered device. For contactless charging, the portable device receiving coil is put in close proximity with the power supply stand transmitting coil to supply power from the transmitting coil to the receiving coil. The internal battery is charged by power induced in the receiving coil. In this method of supplying power, power can be transmitted to the portable device in a contactless manner and there is no need to connect the portable device to the power supply stand via connectors.
This method of supplying power has the drawback that if a metal foreign object such as a paper clip is on the power supply stand when it is supplying power, current will be induced in the foreign object and heat will be generated due to Joule-heating. Further, since wasted power is consumed by current induced in the foreign object, this method also has the drawback that power cannot be efficiently supplied from the power supply stand to the portable device. To eliminate these drawbacks, the charging pad cited in JP 2008-17562 has many temperature sensors disposed in an x-y array on its upper surface to detect foreign objects. The temperature sensors detect heat generated by foreign objects on the charging pad.
If alternating current (AC) power is supplied to the transmitting coil of this charging pad with a metal foreign object on its upper surface, heat will be generated by current induced in the foreign object and that heat will be detected by a temperature sensor disposed nearby.
Since the charging pad described above requires temperature sensors to detect foreign objects and decision circuitry to judge the presence of foreign objects from temperature detected by the temperature sensors, it has the drawbacks of complex circuit structure for foreign object detection and high manufacturing cost. It also has drawbacks such as the inability to stably and reliably detect a foreign object positioned at a distance from the nearest temperature sensor, or delayed detection resulting in detrimental conditions such as excessive heat generated by the foreign object. Further, since this charging pad judges the presence of a foreign object by detecting its heat generation with temperature sensors, the foreign object can be heated to an excessive temperature at the time of detection, and this has the drawback that the foreign object cannot be detected safely.
Since a contactless method of supplying power transmits power from a transmitting coil to a receiving coil by magnetic induction, power transmission efficiency can drop if the relative position of the receiving coil shifts with respect to the transmitting coil. For example, this situation can result from position shift in the portable device due to a disturbance such as vibration during power transmission from the power supply stand to the portable device. If power transmission efficiency drops, power received by the portable device will drop, and communication between the portable device and power supply stand can also become difficult making normal power supply impossible.
The present invention was developed with the object of further resolving the drawbacks described above. Thus, it is one object of the present invention to provide a contactless method of supplying power that can stably and reliably detect foreign objects with a simple circuit structure and low manufacturing cost. It is another object of the present invention to provide a contactless method of supplying power that can rapidly detect the presence of a foreign object. Further, it is another object of the present invention to provide a contactless method of supplying power that can determine abnormal shift in the relative position of the receiving coil with respect to the transmitting coil.